Pyridine-2,3-dicarboxylates are useful intermediates in the preparation of highly effective herbicidal 2-(2-imidazolin-2-yl)nicotinic acids, esters and salts. Said herbicidal agents and methods for their preparation are disclosed In U.S. Pat. Nos. 4,798,619 and 4,758,667. Imidazolinyl nicotinates and derivatives thereof are highly effective herbicides at low rates of application and demonstrate selective control of noxious weeds in the presence of important agronomic crops while exhibiting exceptionally low mammalian toxicity.
Processes for the preparation of substituted pyridine-2,3-dicarboxylates are described in U.S. Pat. Nos. 4,723,011; 4,871,859; 4,948,896 and 5,008,392 among others, however, there is still a need in the art for yet more suitable methods of preparation of these important intermediates.
It is an object of this invention to provide a method for the preparation of substituted and unsubstituted-2,3-pyridinedicarboxylates utilizing dialkyl chloromaleate or dialkyl chlorofumarate or a mixture thereof, an ammonium salt, optionally in the presence of ammonia and an appropriately substituted .alpha.,.beta.-unsaturated aldehyde or ketone.